(1) Field of the Invention
This invention relates to a ground-speed Doppler radar for vehicles which is carried on vehicles such as automobiles and used for detecting the ground speed thereof.
(2) Description of the Prior Art
For the anti-skid braking system of an automobile and other speed controls thereof, it is necessary to precisely and quickly measure the speed of the automobile relative to the ground. As means for measuring the ground speed of the automobile, apparatus is known wherein a microwave Doppler radar is carried on the automobile and the frequency shift or Doppler effect between waves radiated from the antenna of the radar towards the ground and waves reflected from the ground to the antenna is utilized to measure the ground speed of the automobile.
In such ground-speed Doppler radar, microwave signal interference must be prevented from occurring between the waves mentioned above and waves generated from a transceiver carried on an automobile, a high-power wireless equipment situated near the road, or the like.
Experiments have revealed that the interference between the waves of two Doppler radars for measuring the speeds of automobiles takes place only when the difference in transmission frequencies approaches about 10 kHz, and when the waves are identical in the direction of radiation (leakage) and sufficiently high in power. Therefore, the probability of such interference is low.
However, in the case where a high reliability is required, the influence of the microwave signal interference on the accurate detection of speed is not negligible. Further, when the existence of more powerful disturbing waves is considered, effective means for eliminating the microwave signal interference becomes necessary.